Recently, as electronic parts and component embedding technologies have been developed, and electronic appliances have become gradually light, compact, and slim, the demand of a multi-layer flexible printed circuit board (FPCB) is steadily growing.
The multi-layer FPCB has strong heat-resistant, bending-resistant, and medicine-resistant characteristics, enables high-concentration interconnections, and is easily assembled with a device, so the multi-layer FPCB is widely used for cameras, computers, peripheral devices of the computers, mobile communication terminals, video and audio appliances, liquid crystal displays, satellite devices, medial devices, etc.
In general, in order to manufacture the multi-layer FPCB, a dry film is laminated on a copper foil laminated film including a copper foil on a base film, and a predetermined circuit pattern is formed on the dry film through exposure, development, and etching processes. After that, a coverlay film is bonded to the resultant structure.
FIG. 1 is a sectional view showing the structure of the conventional multi-layer FPCB 10.
Referring to FIG. 1, the multi-layer FPCB 10 includes a base film 11, a copper foil layer 12, a via hole 14, plating layers 15 and 16, and an upper film 18.
The base film 11 includes a polyimide film. After gluing the copper foil layer 12 to the upper surface of the base film 11 by an adhesive agent 13, a dry film is laminated on the copper foil layer 12, and then, and exposure, development, and etching processes are performed, thereby forming a predetermined circuit pattern.
After forming the via hole 14 in the base film 11 and the copper foil layer 12, an electroless copper plating layer 15 is formed in the via hole 14 and top surface of the copper foil layer 12 and the via hole 14, and an electrolytic copper plating layer 16 is formed in the via hole 14 and top surface of the electroless copper plating layer and the via hole 14.
The upper film 18 including polyimide adheres to the top surface of the electrolytic copper plating layer 16 by the adhesive 17.
An anisotropic conductive film (ACF) is fusion-welded on the top surface of the upper film 18 of the FPCB formed through the above-described manufacturing process, so that various kinds of electronic appliances are electrically connected to the upper film 18. The ACF includes a conductive adhesive layer having the shape of a thin film formed by mixing minute particle type conductive particles such as metal-coated plastic particles and metal particles, adhesive agents, and additives with each other.
Such an ACF is thin fusion-welded on the top surface of the upper film 18, pressed with a high temperature, and then bonded to the terminals of the electronic appliances, thereby protecting the conductive particles distributed in the ACF between the top surface of the upper film 18 and the terminals of the electronic appliances so as to obtain conductivity. In addition, since the top surface of the upper film 18 and the terminals of the electronic appliances, which are adjacent to each other, are filled with adhesive agents, the conductive particles independently exist. Accordingly, a high insulating characteristic can be obtained.
However, when the FPCB is connected to electronic appliances such as camera modules by using the ACF, the thickness of the FPCB and the ACF makes it difficult to perform the thin bonding between the FPCB and the electronic appliances. In addition, since a heating temperature, a pressing pressure, and a bonding time interval increase proportionally to the thickness of the FPCB, a bad influence may be exerted on the camera module. Accordingly, the defect rate of the camera modules may increase.